Nonreciprocal phase shift fiber-optic gyrometer

ABSTRACT

The present invention relates to a fiber-optic gyrometer including a Sagnac interferometer using two light waves propagating in opposite directions in a interferometer loop including a photodetector which delivers an electrical signal Ud representing the light intensity of the interference between the two waves, and for optically phase-shifting the waves controlled by a square-wave modulation signal Um suitable for controlling an optical phase variation at a frequency FO substantially equal to 1/(2 t   o ), where t o  is the propagation time of a wave through the guide. The photodetector is connected to at least a first sampling circuit and a second sampling circuit which are controlled in phase opposition by a clock. The gyrometer includes a means for inverting the phase of the clock at a frequency f, which is very much less than the frequency FO.

The present invention relates to fiber-optic gyrometers and, moreparticularly, to a device for measuring a non-reciprocal phase shiftgenerated in an optical ring interferometer of the gyrometer, alsoreferred to as a Sagnac interferometer.

Such an interferometer principally includes a source of light energy,generally consisting of a laser, an optical device consisting either ofa certain number of mirrors or of an optical fiber coiled on itself,this device forming an interferometer loop; a device for splitting andmixing of the light and a device for detection and for processing of thedetected signal.

It is known that, in these interferometers, there are two waves whichoriginate from the splitter device and travel in opposite directionsalong the same optical path.

One fundamental property of ring interferometers is reciprocity, whichcan be expressed as follows: any perturbation of the optical pathaffects both waves similarly, even though these two waves do notexperience it at exactly the same time or in the same sense.

There are however, two types of perturbations which violate reciprocity.

These are, on one hand, perturbations which vary in the course of time,and do so on a time scale comparable with the time that the waves taketo propagate along the optical path of the interferometer; and, on theother hand, so-called “non-reciprocal” perturbations, that is to sayperturbations which do not have the same effect on the waves dependingon whether they are propagating along the optical path in one directionor in another. These are physical effects which break the symmetry ofthe medium through which the waves are propagating.

Two known effects exhibit the latter type of perturbation:

-   -   the Faraday effect, or collinear magneto-optical effect, by        which a magnetic field creates a preferential orientation of the        spin of the electrons of an optical material;    -   and the Sagnac effect, or relativistic inertial effect, in which        the rotation of the interferometer with respect to a Galilean        reference frame breaks the symmetry of the propagation time.        This effect is employed to produce gyrometers in particular.

Without the manifestation of “non-reciprocal” perturbations, the phasedifference (which will be referred to below as Δφ) between the twowaves, which are recombined in the splitting and mixing device afterhaving traveled along the optical path, is zero. The detection andprocessing device detects signals representing the optical power of thecomposite wave obtained after recombination. This power can be resolvedinto two components in the interferometers of the prior art:

a constant component and a component proportional to cos (Δφ), thiscomponent existing only when “non-reciprocal” perturbations occur.

If the intent is to measure small-amplitude perturbations, for examplelow speeds of rotation in the case of gyrometers, the componentcontaining the term in cos (Δφ) varies little since the phase shift isclose to zero.

It is then necessary to artificially introduce a fixed additional phaseshift, or “non-reciprocal bias”, in order to increase the sensitivity ofthe measurement. One particularly interesting case is that in which thenew measured phase shift is Δφ′=Δφ+π/2.

In this case, the sensitivity is a maximum since the term to be measuredis proportional to cos (Δφ+π/2), i.e. to sin (Δφ).

Although attractive, this method is confronted with implementationdifficulties and, in particular, the ability to produce a device whichintroduces a “non-reciprocal bias” that is sufficiently stable to beusable.

The instability of these devices is in general of the same order ofmagnitude as the variations of the quantity to be measured.

In order to overcome these drawbacks, Patent FR-B-2 471 583 has thusproposed a phase modulation of the waves which propagate through thering, alternately by +π/2 and −π/2 radians.

This method is based on the property which the Sagnac interferometer hasof producing the equivalent of a discrete time derivation.

Specifically, when a phase modulation is produced at one end of thefiber loop, one of the waves experiences the modulation at the time whenit is produced, while the other wave experiences it with a delay equalto the propagation time through the fiber. This propagation timesatisfies the relation: t_(o)=n1/c, in which n is the refractive indexof silica, 1 is the length of the fiber and c is the velocity of lightin a vacuum. The “natural frequency” of the interferometer is1/(2/t_(o)) and represents the modulation frequency at which both wavesexperience two phase shifts in phase opposition. The phase shift betweenthe two optical waves is therefore equal to the differenceS(t)−S(t)−t_(o)), where S(t) is the signal applied to the phasemodulator. It can therefore be seen that, if the half-period of themodulation signal is t_(o), the phase shift at the output of theinterferometer is equal to two times the applied phase shift. It is thismethod which is used to create the bias giving the working point of theinterferometer.

Added to this phase shift is a phase shift Δφ_(o) due to thenon-reciprocal effect, in the case in point due the rotation if it isnon-zero.

It is possible to process the signals directly and measure the componentin cos (Δφ+π/2).

A more precise method, which avoids the errors due to possible drifts ofthe various elements being used, for example the opto-electronicelements, consists in an indirect method or “zero method”. According tothis method, this difference of the phase shift from ±π/2 radians iscompared by generating an additional phase shift, which is equal inabsolute value to the amplitude of the phase shift due to thenon-reciprocal effect and has an opposite sign, so as to cancel it out.

In order to do this, it is not in practice possible to use the samephysical effect as the one which produces the non-reciprocal effect, inthe case in point altering the rotation.

Electrical means are employed in order to generate a feedback signal. Itis assumed that greater control can be obtained over these electricalmeans than over the other elements of the interferometer, which isconfirmed by experience.

The purpose of this feedback is to create a phase shift between the twowaves which is constantly equal and of opposite sign to that induced bythe speed of rotation. If the speed is constant and creates a phaseshift Δφ, it is therefore necessary for the instantaneous value of thephase modulation to have varied by (Δφ_(o)+2πn) radians between twotimes separated by t, n being an integer. This is therefore theequivalent of an integral of the velocity. One procedure is to generatea phase “ramp” with a slope proportional to (Δφ/t_(o)).

This method, however, entails two separate operations: the phasemodulation and the generation of a feedback signal. Furthermore, thefactor of proportionality or scale factor is not related to the oneemployed for the modulation by ±(π/2) radians.

Furthermore, the phase “ramp” cannot be infinite, that is to say thesignal, which consists in practice of a control voltage of a phasemodulator, cannot increase above a specific threshold.

One usable method is to generate sawtooth phase-shift control signalswith a peak-to-peak amplitude of 2π radians, the mathematical functionsinvolved being periodic and having a period of 2π radians. This leadsthe problem of precisely determining this phase-shift amplitude equal to2π radians.

The phase “ramp” consists of a digital signal. The phase modulation,which is also in digital form, and this phase “ramp” are combined toform a single signal, and converted into an analog control signal for aphase modulator arranged in the ring.

An interferometer operating on this principle is described in Patent FR2 566 133. However, the interferometer described in Patent FR 2 566 133contains electronic circuits, for processing the detected optical power,which introduce drifts that falsify the rotation measurement in the longterm. Gyrometers are very sensitive to the drifts since rotations aremeasured over long times. Any drift of the electronic circuits can leadto a time-integrated signal drift. In order to avoid these drifts, ithas already been proposed to digitize the signal immediately at theoutput of the photodetector, and subsequently to process everythingdigitally, but this technique has the drawback of requiring ananalog/digital converter that has a very large dynamic range.

In order to overcome the drawbacks of the prior art which have just beendescribed, the invention provides a fiber-optic gyrometer comprising aSagnac interferometer using two light waves propagating in oppositedirections in a interferometer loop, comprising a photodetector whichdelivers an electrical signal representing the light intensity of theinterference between the two waves, and means for opticallyphase-shifting the waves controlled by a square-wave modulation signalsuitable for controlling an optical phase variation at a frequency FOsubstantially equal to 1/(2t_(o)), where t_(o) is the propagation timeof a wave through the guide, the photodetector being connected to atleast a first sampling circuit and a second sampling circuit which arecontrolled in phase opposition by a clock with frequency FO and providetwo samples in each period, respectively on a first input and a secondinput of a differential amplifier, an analog-digital converter at theoutput of the differential amplifier and an adder/subtractor foraccumulating the digital values successively provided by theanalog-digital converter, the adder/subtractor providing a contentrepresenting a parameter of the rotation measurement of the gyrometer,characterized in that a means is provided for inverting the phase of theclock at a frequency f, which is very much less than the frequency FO,so as to alternate the sense of the difference of samples at the outputof the differential amplifier at the frequency f, and in that theadder/substractor is also controlled by the frequency f in order tofunction alternately as an adder or as a subtractor.

The invention will be understood more clearly, and other advantages willbecome apparent, with the aid of the following description and theappended drawings, in which:

FIG. 1 represents a ring interferometer of the prior art;

FIG. 2 represents the variation of the optical power P_(S) in an outputbranch of the interferometer in FIG. 1.

FIG. 3 represents a block diagram of a gyrometer according to theinvention.

FIG. 4 a represents a modulation signal Um of the phase modulator of thegyrometer in FIG. 3.

FIG. 4 b represents the control signal of an invertor of thecomplementary states of a clock of the gyrometer in FIG. 3.

FIG. 5 shows a variant of the gyrometer in FIG. 3 according to theinvention, which corrects the scale factor of the phase ramp controllingthe modulator.

It is firstly useful to recall the principal phenomena involved in aring interferometer of the Sagnac type, as well as the modulation methodtaught by the aforementioned Patent FR-B-2 471 583.

FIG. 1 schematically illustrates the architecture of a ringinterferometer as described in that patent.

A laser source S produces a beam of parallel rays 1 in the direction ofa splitter device consisting, for example, of a plate or asemitransparent mirror M optically coupled to the ring 2 of theinterferometer. This ring 2 may be produced, for example, with the aidof a single-mode fiber coiled on itself. The reason is that thesensitivity of the measurement is increased owing to the use of a longoptical path, which is proportional to the number of turns. This ring 2is looped back onto the splitter device M, which also fulfils the roleof a mixer device and thus defines an output branch 3. Two wavespropagating in opposite directions therefore travel through the ring:one in the clockwise direction S1 and the other in the counterclockwisedirection S2. These two waves are recombined on the splitter plate M.The result of this recombination can be observed in the output branch 3with the aid of the photodetector 4.

Let Δφ_(o) be the phase difference between the two waves propagating inopposite directions in the ring, and let P_(S) be the output opticalpower which can be measured in the output branch 3. In the absence ofany “non-reciprocal” perturbation, Δφ_(o) is zero.

If, by way of nonlimiting example, a gyrometer employing a ringinterferometer is considered, a “non-reciprocal” perturbation will becreated by the rotation of the gyrometer. The phase difference is nolonger zero, and Δφ_(o)=αΩ, where Ω is the speed of rotation andα=kL/λC, where k is a constant that depends on the geometry of thegyrometer, L is the length of the optical path, λ is the wavelength ofthe light emitted by the laser source, and C is the velocity of light inthe ring 2. When the speed of rotation Ω increases, the phase differenceΔφ_(o) increases proportionately because the coefficient α remainsconstant. The optical power P_(S) changes according to a cosinusoidallaw. Specifically:

P_(S)=P_(1S)+P_(2S)+2P_(1S)P_(2S) cos (Δφ_(o)); in which relation thecomponent P_(1S) corresponds to the direction S1 and the componentP_(2S) corresponds to the direction S2. The sensitivity of themeasurement for a given value Δφ is expressed by the derivative ofP_(S):dP _(S) /d(Δφ_(o))=−2P _(1S) P _(2S) sin (Δφ_(o)).

The sensitivity of the interferometer is very low if the phasedifference Δφ differs little from zero. This is the case in a gyrometerif the intent is to measure low speeds of rotation Ω. The variation ofthe optical power P_(S) in the output branch as a function of the phasedifference Δφ is illustrated by the diagram in FIG. 2.

The terms P_(1S) and P_(2S) may be assumed to be equal. This means thatthe detected power is a minimum for a phase difference Δφ=π radians. Itpasses through a maximum at Δφ=0 and at 2π radians, and so on.

In order to increase the sensitivity of the interferometer, a constant“non-reciprocal bias” may be introduced into the phase of the two wavespropagating in opposite directions, so as to move the working point ofthe interferometer.

In the case of a function which varies according to a cosinusoidal law,the point of highest sensitivity is obtained by the angles (2n+1)π/2radians, with n being an integer. A bias may therefore be selected whichintroduces a phase variation for each wave with an absolute value of π/4radians but with opposite signs. In the absence of any “non-reciprocal”perturbation, the phase difference is then at the point P_(SO) in FIG.2.

According to the teaching of the aforementioned French patent, a phasemodulator 5 incorporating a reciprocal effect is introduced into thepath of the waves through the ring 2. The phase modulator 5 (FIG. 1) isstimulated so as to create a phase variation Φ(t) of the wave whichpasses through it. This variation is periodic, its period being equal to2t_(o), t_(o) being the propagation time of a wave in the ring.

FIG. 3 represents the architecture of a gyrometer according to theinvention, employing the ring interferometer in FIG. 1.

An electronic device 20 for operating the interferometer receiveselectrical information from the photodetector 4 optically coupled to thechannel 3, at the output of the splitter plate M of the interferometerin FIG. 1, and provides a modulation signal Um to the phase modulator 5introduced into the ring 2 of said interferometer.

The photodetector 4 converts the output optical intensity of the mixerdevice M (splitter plate M) into an electrical voltage Ud which isapplied, through an amplifier 24, to a synchronous detection circuit 26driven by two additional control signals Ca and Cb with the modulationfrequency FO=1/(2t_(o)).

A processor 28 manages all of the electronic operating device 20 of thegyrometer according to the invention. The control signals Ca and Cbneeded for controlling the synchronous detector 26 may, for example, becopies of the pulse signals of a clock 30 driven by a quartz oscillator32 of the processor 28.

The synchronous detector 26 essentially includes a first sample/holdcircuit 34 and a second sample/hold circuit circuit 36 which arecontrolled by the clock 30, through an invertor 38 of the complementarylogical states H and H provided by the clock 30, so that the outputsignal Ud of the amplifier 24 is sampled by the first sample-and-holdcircuit 34 during one half-period t_(o) of the phase modulation of theoptical signal in one sense, then by the second sample-and-hold circuit36 during the other, subsequent half-period of the phase modulation ofthe optical signal in the other sense, the two senses corresponding tothe synchronous phase modulations by +n/2 and −n/2 (sum of the physicalphase shifts along S2 by + and −n/4 at T−t_(o) and along S1 by − and+n/4 at T).

When the gyrometer is in rotation, the entire curve of theinterferometer as a function of the phase shift applied by the modulatoris displaced. This produces a modulation of the output voltage Ud of thephotodetector with the frequency FO=1/(2t_(o)), the amplitude of whichis proportional to the speed, if the latter is small enough for thephase shift to remain in the linear region of the response curve.

The amplitude of this modulation is extracted by the synchronousdetector 26, which provides an analog voltage Us corresponding to thephase variation. The analog voltage Us is applied, after digitizing byan analog/digital converter 42, to a digital control circuit 44 whichgenerates a composite signal for modulation of the phase modulator 5.The analog/digital converter 42 is controlled by a clock with frequencyFO.

The purpose of the digital control circuit 44 is to construct a digitalramp and to combine it with the digital phase-modulation signals. Tothis end, the digital control circuit 44 includes an adder/subtractor 46which receives, on inputs, the output digital signals of theanalog-digital converter 42 and an accumulation instruction with thefrequency FO and provides, at an output, digital information to anintegrator 48 tasked with producing a digital ramp whose slope is afunction of the speed of rotation of the gyrometer.

The digital output of integrator 48 addresses a digital/analog converter50 which, through a power amplifier 52, generates the analog voltage Umfor modulation of the phase-shifter 5 arranged in the path of the lightwaves of the interferometer.

According to the principal characteristic of the gyrometer according tothe invention, the operating device includes a means for inverting thephase of the clock 30 at a frequency f, which is very much less than thefrequency FO, so as to alternate the sense of the difference of samplesat this frequency f. The adder/subtractor 46 is also controlled by thefrequency f in order to function alternately as an adder (+1) or as asubtractor (−1). To this end, the states H and H provided by the clock30 are inverted by the invertor 38, at the rate of the frequency fapplied to a control input 54 of the invertor 38.

The sample-and-hold circuits 34, 36 sample the output signal Ud of theamplifier 24 of the photodetector 4. The signal Ud represents theoptical power resulting from the interference between the two lightwaves S1 and S2 propagating through the optical fiber of theinterferometer.

Each of the outputs 60, 62 of the sample-and-hold circuits 34 and 36addresses one or other of the two inputs (+, −) of the differentialamplifier 64 which delivers, at its output, the voltage Us representingthe difference between two consecutive samples taken during one or otherhalf-period of the frequency of FO of the optical power signal Ud at theoutput of the photodetector 4.

Each of the sample-and-hold circuits of the synchronous detector hascontrol inputs Ea, Ēa and Eb, Ēb driven by the clock 30 through theinvertor 38, as described below. The output Ca of the invertor isconnected respectively to the input Ēa of the first sample-and-holdcircuit 34 and to the input Eb of the second 36, and the output Cb ofthe invertor 38 is connected to the input Ea of the firstsample-and-hold circuit 34 and to the input Ēb of the second; hence, inthe known way, the sample which is held is the analog value present atthe input on the leading edges, for example, of the inputs Ea and Eb.The signal presented to the input of the analog/digital converter 42when controlling the analog/digital conversion is the difference betweenthe values sampled during the last leading edges of the signals at theinputs Ea and Eb.

The invertor 38 receives through its control input 54 an inversioncontrol signal Co with the frequency f, which has a high state during ahalf-period of duration ½.f then a low state during the subsequenthalf-period of the same duration. The states H and H are thustransmitted respectively to the outputs Ca and Cb of the invertor 38when the inversion control signal is in the high state, for example, theoutput Ca transmitting the state H of the clock and the output Cbtransmitting the state H, and the states H and H are inverted at theoutputs Ca and Cb of the invertor 38 when the inversion control signalis in the low state, the output Ca transmitting the state H of the clockand the output Cb transmitting the state H.

During the high state of the control signal Co of the invertor 38, thedifferential amplifier 64 presents a voltage at its output correspondingto a sequence of differences of two consecutive samples A_(p) andB_((p+1)) taken respectively during one or other modulation phase of thelight signals (+n/2 and −n/2). The output signal Us of the differentialamplifier, representing the difference of the samples (A_(p)−B_((p+1)))during a period 2to 2t_(o), is applied to the adder/subtractor 46 afterdigitizing by the analog/digital converter 42.

While the control signal of the invertor 38 is in the high state, theadder/subtractor 46 is controlled by the processor 28 so as to carry outpositive accumulation (+1).

When the control signal of the invertor changes from the high state tothe low state, the states H and H are inverted at the outputs Ca and Cbof the invertor 26, the output Ca transmitting the state H of the clockand the output Cb transmitting the state H, inverting the logical statesat the respective inputs Eb and Ēb Ea and Ēa of the sample-and-holdcircuits. The samples taken by the first sample-and-hold circuit 34,when the control signal Co of the invertor was in the high state duringa phase variation of the light signals in one sense, are taken by thesecond sample-and-hold circuit 36 when the control signal Co of theinvertor 38 is in the low state, and vice versa. Since the difference ofthe output samples of the differential amplifier changes sign, theprocessor 28 inverts the control of the adder/subtractor 46 in order tocarry out negative accumulation (−1) and hence keep the same sense ofthe phase ramp.

FIG. 4 a represents a modulation signal Um of the phase modulator 5 ofthe gyrometer in FIG. 3 according to the invention, and FIG. 4 brepresents the control signal Co of the state invertor of the clock 30.

The modulation signal Um applied to the phase modulator 5 generates thephase ramp, with a slope proportional to (φ_(o)/t₀) and a peak-to-peakamplitude equal to 2π radians, which is combined with the phasemodulation signal +π/2 and −π/2 with the frequency ½.t₀.

In FIG. 4 a, the samples taken by the first sample-and-hold circuit 34are identified by the letter A and those taken by the secondsample-and-hold circuit 36 are identified by the letter B.

Assuming that the control signal Co of the invertor is in the high state(state 1 in the figure) before an instant x1 during the phase ramp, thesamples A are taken during the half-period of duration t_(o) giving riseto a phase modulation by +π/4, and the samples B are taken during theother half-period giving rise to −π/4 phase modulation. After theinstant x1, the control signal Co of the invertor changes state,entering the low state (state 0 in FIG. 4 b) inverting the way in whichthe samples are taken, the samples A then being taken during the halfperiod of duration t_(o) giving rise to a phase modulation by −π/4, andthe samples B being taken during the other half-period giving rise to+π/4 phase modulation.

The role of this alternation function is to abruptly invert thesynchronous detection phase and simultaneously alternate the sign of theaddition function of the first accumulator of the signal coming from theanalog-digital converter.

The adder/subtractor 46 will thus, for example, perform the followingaddition:

-   -   (A1−B2)+(A3−B4)+ . . . (A997−B998)−(1000−B1001) . . .        −(A1998−B1999)+(A2001−B2002)+ . . .

With a positive sign for the difference and positive accumulation (+1)from A1 to B998, then a sign change of the difference, which becomesnegative, and negative accumulation (−1) from A1000 to B1999, thenanother sign change of the difference, which becomes positive, andpositive accumulation (+1), and so on.

The slaving function is not modified since the sign of the phase errorsignal accumulated in the first accumulator is not affected by thedouble inversion. Only the sign of the error of the offset of thevoltages of the electronics is alternated because this error is notmodified by the first inversion (phase inversion), while it is by thesecond inversion (of sign).

The operation sequence carried out by the adder/subtractor 46 shows thatone sample x of the signal Ud has been omitted at each transition of theclock f. The average rate of the operations of the adder/subtractor 46is equal to FO−f. In the event of rotation with constant sign, this mayentail a systematic error with a relative value of f/FO. In order tocorrect this error, the average value of addition/subtraction precedingthe transition of the clock f may be stored, and half of this averagevalue may be added/subtracted by an additional circuit or logicaloperator in order to compensate for the missing half-sample.

The operating device of the gyrometer according to the invention alsomakes it possible to use analog-digital conversion that is notnecessarily at the frequency of the optical phase modulation, whichreduces the power consumption of the electronics, improves the noiseimmunity and compensates for the offset defects of the analog part ofthe electronics, especially the voltage offsets of the differentialamplifiers.

Patent FR 8409311 proposes to correct the scale factor of the phase rampby comparing two modes of operation, corresponding to the phase shiftsπ/2 and 3π/2, of the phase modulator.

FIG. 5 shows the layout of a variant of the gyrometer in FIG. 3according to the invention, which corrects the scale factor of the phaseramp of the phase modulator 5.

In this variant, the operating device of the gyrometer includes foursample-and-hold circuits. A first group of two sample-and-hold circuits70, 72 addresses the two inputs of a first differential amplifier 74,the assembly forming a first synchronous detector 75, the firstdifferential amplifier 74 providing the difference of the samples takenby the first group of sample-and-hold circuits.

A second group of two other sample-and-hold circuits 76, 78 addressedthe two inputs of a second differential amplifier 80, the assemblyforming a second synchronous detector 79, the second differentialamplifier providing the difference of the samples taken by the secondgroup of sample-and-hold circuits.

The first and second groups of sample-and-hold circuits are respectivelydriven by a first clock 82 and a second clock 84 through respectiveinvertors 86, 88 of the states of the clocks, according to the operationdescribed for the case of the gyrometer in FIG. 3.

The first and second groups of sample-and-hold circuits operate assynchronous detectors in the same way as for the case of the gyrometerin FIG. 3 described above. To this end, the processor 28 simultaneouslycontrols the following, depending on whether it is operating with aphase modulation interval of π/2 or 3π/2:

-   -   a first switch 88 which selects either the output of the first        differential amplifier 74 of the first group of sample-and-hold        circuits, or the output of the second differential amplifier 80        of the second group of sample-and-hold circuits, in order to        address the analog-digital converter 42 of the electronic        operating device;    -   the clock 82, 84 and the state invertor 86, 88 which are        associated with the selected group of sample-and-hold circuits;    -   a second switch 92, of the same type as the first switch 88,        which provides information to the digital/analog converter 50 on        the basis of the output information G1 and G2 of the first and        second groups of sample-and-hold circuits.

1. A fiber-optic gyrometer, comprising: a Sagnac interferometer usingtwo light waves propagating in opposite directions in an interferometerloop; a photodetector which delivers through an adaptor an electricalsignal Ud representing the light intensity of the interference betweenthe two light waves; phase modulator means for optically phase-shiftingthe light waves controlled by a square-wave modulation signal Umsuitable for controlling an optical phase variation at a frequency FOsubstantially equal to 1/(2t_(o)), where t_(o) is the propagation timeof a light wave through the interferometer loop, the photodetector beingconnected to a first sampling circuit and a second sampling circuitwhich are controlled in phase opposition by a clock with a frequency FOand provide two samples in each period, respectively, on a first inputand a second input of a differential amplifier; an analog-digitalconverter at the output of said differential amplifier; and anadder/subtractor for accumulating the digital values successivelyprovided by the analog-digital converter, said adder/subtractorproviding a content representing a parameter of the rotation measurementof the gyrometer; means for inverting the phase of the clock at afrequency f, which is very much less than the frequency FO, so as toalternate the sense of the difference of samples at the output of thedifferential amplifier at the frequency f, the adder/subtractor beingalso controlled by the frequency f so that the adder/subtractor worksalternately in adder or in subtractor.
 2. The fiber-optic gyrometer asclaimed in claim 1, wherein the modulation amplitude of interferencebetween the two waves is extracted by a synchronous detector formed bythe first sampling circuit and the second sampling circuit connectedrespectively to the first input and the second input of the differentialamplifier to provide an analog voltage Us corresponding to the phasevariation between the waves, the analog voltage Us being applied, afterdigitizing by the analog/digital converter, to a digital control circuitincluding the adder/subtractor which receives, on inputs, the outputdigital signals of the analog-digital converter, and provides at anoutput, digital information to an integrator and that generates acomposite signal for modulation of the optical phase-shifting means,said digital control circuit constructing a digital ramp combined withthe digital phase-modulation signals.
 3. The fiber-optic gyrometer asclaimed in claim 2, further including a synchronous detection circuitdriven by two complementary control signals Ca and Cb with themodulation frequency FO=1/(2t_(o)), the synchronous detector including afirst sample-and-hold circuit and a second sample-and-hold circuit whichare controlled by the clock, through an invertor of the complementarylogical states H and H provided by the clock, so that the output signalUd of the adaptor is sampled by the first sample-and-hold circuit duringone half-period t_(o) of the phase modulation of the optical signal onehalf-period t_(o), then by the second sample-and-hold circuit during theother, subsequent half-period of the phase modulation of the opticalsignal.
 4. The fiber-optic gyrometer as claimed in claim 2, wherein themodulation signal Um applied to the phase modulator generates the phaseramp, with a slope proportional to (φ_(o)/t₀) and a peak-to-peakamplitude equal to 2π radians, which is combined with the phasemodulation signal +π/2 and −π/2 with the frequency ½t₀.
 5. Thefiber-optic gyrometer as claimed in claim 1, further including asynchronous detection circuit formed by the first sampling circuit andthe second sampling circuit both connected to the differentialamplifier, the first sampling circuit and the second sampling circuitbeing sample-and-hold circuits, the synchronous detection circuit drivenby two complementary control signals Ca and Cb with the modulationfrequency FO=1/(2t_(o)), which are controlled by the clock that providecomplementary logical states H and H, through an invertor, so that theoutput signal Ud of the adaptor is sampled by the first sample-and-holdcircuit during one half-period t_(o) of the phase modulation of theoptical signal, then by the second sample-and-hold circuit duringsubsequent half-period of the phase modulation of the optical signal. 6.The fiber-optic gyrometer as claimed in claim 5, wherein the modulationsignal Um applied to the phase modulator generates the phase ramp, witha slope proportional to (φ_(o)/t₀) and a peak-to-peak amplitude equal to2π radians, which is combined with the phase modulation signal +π/2 and−π/2 with the frequency ½t_(o).
 7. The fiber-optic gyrometer as claimedin claim 5, wherein the digital control circuit includes anadder/subtractor which receives, on inputs, the output digital signalsof the analog-digital converter and an accumulation instruction with thefrequency FO and provides, at its output, digital information to anintegrator tasked with producing a digital ramp whose slope is afunction of the speed of rotation of the gyrometer, the digital outputof integrator addressing a digital/analog converter which, through apower amplifier, generates the analog voltage Um for modulation of thephase-shifter arranged in the path of the light waves of theinterferometer.
 8. The fiber-optic gyrometer as claimed in claim 5,wherein each of the sample-and-hold circuits of a synchronous detectioncircuit has control inputs Ea, Ēa and Eb, Ēb driven by the clock throughthe invertor, one output Ca of the invertor being connected respectivelyto the input Ēa of the first sample-and-hold circuit and to the input Ebof the second, and the output Cb of the invertor being connected to theinput Ea of the first sample-and-hold circuit and to the input Ēb of thesecond, the sample which is held being the analog value present at theinput of the inputs Ea and Eb, the signal presented to the input of theanalog/digital converter when controlling the analog/digital conversionbeing the difference between the values sampled during the last leadingedges of the signals at the inputs Ea and Eb.
 9. The fiber-opticgyrometer as claimed in claim 8, wherein the invertor receives throughits control input an inversion control signal Co with the frequency f,which has a high state during a half-period of duration f/2 then a lowstate during the subsequent half-period of the same duration, and inthat states H and H are transmitted respectively to the outputs Ca andCb of the invertor when the inversion control signal is in the highstate, the output Ca transmitting the state H of the clock and theoutput Cb transmitting the state H, and the states H and H are invertedat the outputs Ca and Cb of the invertor when the inversion controlsignal is in the low state, the output Ca transmitting the state H ofthe clock and the output Cb transmitting the state H.
 10. Thefiber-optic gyrometer as claimed in claim 8, wherein the digital controlcircuit includes an adder/subtractor which receives, on inputs, theoutput digital signals of the analog-digital converter and anaccumulation instruction with the frequency FO and provides, at itsoutput, digital information to an integrator tasked with producing adigital ramp whose slope is a function of the speed of rotation of thegyrometer, the digital output of integrator addressing a digital/analogconverter which, through a power amplifier, generates the analog voltageUm for modulation of the phase-shifter arranged in the path of the lightwaves of the interferometer.
 11. The fiber-optic gyrometer as claimed inclaim 5, wherein the invertor receives through its control input aninversion control signal Co with the frequency f, which has a high stateduring a half-period of duration f/2 then a low state during thesubsequent half-period of the same duration, and in that states H and Hare transmitted respectively to the outputs Ca and Cb of the invertorwhen the inversion control signal is in the high state, the output Catransmitting the state H of the clock and the output Cb transmitting thestate H, and the states H and H are inverted at the outputs Ca and Cb ofthe invertor when the inversion control signal is in the low state, theoutput Ca transmitting the state H of the clock and the output Cbtransmitting the state H.
 12. The fiber-optic gyrometer as claimed inclaim 11, wherein the modulation signal Um applied to the phasemodulator generates the phase ramp, with a slope proportional to(φ_(o)/t₀) and a peak-to-peak amplitude equal to 2π radians, which iscombined with the phase modulation signal +π/2 and −π/2 with thefrequency ½t₀.
 13. The fiber-optic gyrometer as claimed in claim 1,wherein the modulation signal Um applied to a phase modulator generatesa phase ramp, with a slope proportional to (Δφ_(o)/t₀), Δφ_(o) being aphase shift due to the non-reciprocal effect, and a peak-to-peakamplitude equal to 2π radians, which is combined with phase modulationsignal +π/2 and −π/2 with the frequency 1/(2t₀).
 14. The fiber-opticgyrometer as claimed in claim 13, wherein in order to correct the scalefactor of the phase ramp of signal U_(m), an operating device of thegyrometer includes four sample-and-hold circuits, a first group of twosample-and-hold circuits addressing the two inputs of a firstdifferential amplifier, an assembly forming a first synchronousdetector, the first differential amplifier providing the difference ofthe samples taken by the first group of sample-and-hold circuits, and asecond group of two other sample-and-hold circuits addressing the twoinputs of a second differential amplifier, the assembly forming a secondsynchronous detector, the second differential amplifier providing thedifference of the samples taken by the second group of sample-and-holdcircuits, the first and second groups of sample-and-hold circuits beingrespectively driven by a first clock and a second clock throughrespective invertors of the states of the clocks, the processorsimultaneously controlling the following, depending on whether it isoperating with a phase modulation interval of π/2 or 3π/2: a firstswitch which selects either the output of the first differentialamplifier of the first group of sample-and-hold circuits, or the outputof the second differential amplifier of the second group ofsample-and-hold circuits, in order to address the analog-digitalconverter of the electronic operating device; the clock and the stateinvertor which are associated with the selected group of sample-and-holdcircuits; a second switch, of the same type as the first switch, whichprovides information to the digital/analog converter on the basis of theoutput information G1 and G2 of the first and second groups ofsample-and-hold circuits.
 15. The fiber-optic gyrometer as claimed inclaim 13, wherein the digital control circuit includes anadder/subtractor which receives, on inputs, the output digital signalsof the analog-digital converter and an accumulation instruction with thefrequency FO and provides, at its output, digital information to anintegrator tasked with producing a digital ramp whose slope is afunction of the speed of rotation of the gyrometer, the digital outputof integrator addressing a digital/analog converter which, through apower amplifier, generates the analog voltage Um for modulation of thephase-shifter arranged in the path of the light waves of theinterferometer.
 16. The fiber-optic gyrometer as claimed in claim 1,wherein a processor manages gyrometer, providing the control signals Caand Cb necessary for controlling a synchronous detector.
 17. Thefiber-optic gyrometer as claimed in claim 1, further including asynchronous detection circuit driven by two complementary controlsignals Ca and Cb with the modulation frequency FO−1/(2t₀), thesynchronous detector including a first sample-and-hold circuit and asecond sample-and-hold circuit which are controlled by the clock,through an invertor of the complementary logical states H and H providedby the clock, so that the output signal Ud of the adaptor is sampled bythe first sample-and-hold circuit during one half-period t_(o) of thephase modulation of the optical signal one half-period t_(o), then bythe second sample-and-hold circuit during the other, subsequenthalf-period of the phase modulation of the optical signal.
 18. Thefiber-optic gyrometer as claimed in claim 1, wherein each of thesample-and-hold circuits of a synchronous detection circuit has controlinputs Ea, Ēa and Eb, Ēb driven by the clock through the invertor, oneoutput Ca of the invertor being connected respectively to the input Ēaof the first sample-and-hold circuit and to the input Eb of the second,and the output Cb of the invertor being connected to the input Ea of thefirst sample-and-hold circuit and to the input Ēb of the second, thesample which is held being the analog value present at the input of theinputs Ea and Eb, the signal presented to the input of theanalog/digital converter when controlling the analog/digital conversionbeing the difference between the values sampled during the last leadingedges of the signals at the inputs Ea and Eb.
 19. The fiber-opticgyrometer as claimed in claim 1, wherein control signals Ca and Cb arecopies of pulse signals of a clock driven by a quartz oscillator of theprocessor.
 20. The fiber-optic gyrometer as claimed in claim 19, whereinin order to correct the scale factor of the phase ramp, the operatingdevice of the gyrometer includes four sample-and-hold circuits, a firstgroup of two sample-and-hold circuits addressing the two inputs of afirst differential amplifier, the assembly forming a first synchronousdetector, the first differential amplifier providing the difference ofthe samples taken by the first group of sample-and-hold circuits, and asecond group of two other sample-and-hold circuits addressing the twoinputs of a second differential amplifier, the assembly forming a secondsynchronous detector, the second differential amplifier providing thedifference of the samples taken by the second group of sample-and-holdcircuits, the first and second groups of sample-and-hold circuits beingrespectively driven by a first clock and a second clock throughrespective invertors of the states of the clocks, the processorsimultaneously controlling the following, depending on whether it isoperating with a phase modulation interval of π/2 or π3π/2: a firstswitch which selects either the output of the first differentialamplifier of the first group of sample-and-hold circuits, or the outputof the second differential amplifier of the second group ofsample-and-hold circuits, in order to address the analog-digitalconverter of the electronic operating device; the clock and the stateinvertor which are associated with the selected group of sample-and-holdcircuits; a second switch, of the same type as the first switch, whichprovides information to the digital/analog converter on the basis of theoutput information G1 and G2 of the first and second groups ofsample-and-hold circuits.